This invention relates to a thermal infrared detecting device which detects infrared rays.
Generally, a bolometer type, a pyroelectric type, and a thermocouple (or thermopile) type are known as a thermal infrared detecting element. The thermal infrared detecting element is used as a thermal infrared detecting device singly or together with other thermal infrared detecting elements. For example, a plurality of the thermal infrared detecting elements is used as an array sensor. The array sensor is combined with an optical unit, an amplifying unit, a signal processing unit, a displaying unit etc. to form an infrared imaging device.
The bolometer type thermal infrared detecting element has an infrared absorber, a thermo-resistive converter, and a readout circuit. In this device, the infrared absorber is irradiated by infrared rays which are focused by the optical unit. The infrared absorber absorbs the infrared rays and increases its temperature. This effects a temperature change of the thermo-resistive converter. The thermo-resistive converter changes its resistance with the temperature change. If the thermo-resistive converter is supplied with a predetermined current, a predetermined voltage is been across it. The voltage changes together with the change of the resistance. Namely, the voltage changes with the temperature change which depends on strength of the infrared rays. The readout circuit produces an output signal by the use of the voltage occurred across the thermo-resistive converter.
Therefore, the thermal infrared detecting element can detects the infrared rays and produces the output signal which is representative of the strength of the infrared rays.
However, the thermal infrared detecting element is easily influenced by its operating temperature which is influenced by surrounding temperature. In other words, the output signal includes a drift which depends on the operating temperature.
In order to remove the drift from the output signal, a light chopper is generally combined with the thermal infrared detecting element.
The light chopper chops the infrared rays and intermittently supplies them to the thermal infrared detecting element. As a result, the thermal infrared detecting element produces an alternating signal having an amplitude which is representative of the strength of the infrared rays. Namely, the drift is easily removed from the output signal by removing direct-current component from the output signal.
However, the light chopper is not reliable, and consumes power, because the light chopper needs a motor.
In this field, there have been proposals to solve the above mentioned problems. These proposals provides thermal infrared detecting devices each of which has a pair of the thermal infrared detecting elements (or pixels). One is sensitive to the infrared rays and the other is insensitive to the infrared rays. Each of the thermal infrared detecting devices ideally produces an output signal without the drift by the use of difference between electric characteristics of the sensitive and the insensitive elements.
However, these thermal infrared detecting devices suffer from the following drawbacks.
For example, it is difficult to manufacture. In addition, the thermal infrared detecting device is not sensitive enough to infrared rays. Moreover, it is difficult to completely remove the drift from the output signal.